What is the ruling on zakat for used and stored gold that is bought and sold as needed, and what about past years for which zakat was not paid?

First: Gold acquired for trade is subject to business zakat. However, used gold (jewelry) is not included in this category. It appears from the question that the mentioned gold is used gold.

Second: Jurists differed regarding the zakat on jewelry intended for use, and the preponderant opinion is that zakat is obligatory on it. If you did not pay zakat for past years due to ignorance of its obligation or by following an opinion that it is not obligatory, then there is nothing due from you for those years, and zakat becomes obligatory on you going forward.

However, if you neglected to pay zakat out of heedlessness, then you must pay it now by estimating the amount of gold you owned.

Sheikh Ibn Baz, may Allah have mercy on him, was asked about a woman who owned ornamental jewelry for years and then learned of the obligation of zakat. He replied: "You must pay zakat from the moment you learned of its obligation on jewelry. As for what passed before your knowledge, there is no zakat due from you, because religious rulings only become binding after knowledge." (Fatawa Sheikh Ibn Baz 14/111).

He also said in a similar question: "She must pay zakat on her jewelry annually going forward if it reaches the nisab... As for the past years before she knew of the obligation of zakat on jewelry, there is nothing due from her for them, due to her ignorance and the doubt in the matter, as some scholars do not deem zakat obligatory on jewelry that is worn or prepared for that purpose. However, the preponderant opinion is the obligation of zakat on it if it reaches the nisab and a year has passed, due to the evidence from the Qur'an and Sunnah supporting it." (Fatawa Islamiyyah 2/85).